Machine Fundamentals
The term electrical “machine” is a general term used to include
electric motors and generators. Electric machines are used to perform
electromechanical energy conversion (converting electrical energy into
mechanical energy, or vice versa).
Electric motor - converts electrical energy into mechanical energy.
Electric generator - converts mechanical energy into electrical energy.
The more generic term of electrical machine is necessary since the same
device can operate as a motor or a generator. Each electrical machine has
a stationary component and a moving component. The motion of the
moving machine component can be linear, oscillatory or rotational. The
most common type of motion is rotational. There are three types of rotating
machines.
Types of Rotating Machines
1.
2.
3.
Synchronous machines
Induction (asynchronous) machines
DC machines
Any rotating machine consists of two basic parts. The stationary (outside)
portion of the machine is called the stator. The rotating (inside) portion of
the machine is called the rotor. In between the stator and the rotor is an air
gap. The rotor is typically mounted on a steel rod, called the shaft.
The shaft is connected directly to the rotor. With regard to the
rotational velocity of the machine, the terms “machine speed, “rotor speed”
and “shaft speed” are synonymous. The rotor and the stator each consist
of three components.
Rotor/Stator Components
1.
2.
3.
Core
Windings
Insulation
The core of the rotor or stator is a ferromagnetic material used to effectively
conduct the magnetic fields through the windings (coils). The winding
currents are the sources of magnetic fields in the machine. There are two
types of winding currents and three types of windings.
Winding Currents
1.
2.
Load current - a winding current that varies with load.
Magnetizing (exciting) current - a winding current that
provides a magnetic field.
Winding Types
1.
2.
3.
Armature - carries load current only.
Field winding - carries magnetizing current only.
Primary winding - carries both load and magnetizing
current.
The power requirements of a field winding are relatively small in
comparison to those of a winding that must carry load current. Typically
the input power to a field winding is somewhere between 0.5% to 2% of the
rated machine power. The winding conductors are typically copper or
aluminum and may consist of coils or heavy bars, depending on the current
requirement.
The insulation system of a rotating machine consists of three
components.
Insulation Components
1.
2.
3.
Conductor insulation - insulates the winding conductors,
typically natural or synthetic varnish.
Coil insulation - insulates the coil winding from the core
conductor, usually tape.
Slot liner - for machines with coils located in slots.
The following basic operating information about the rotating machine
is typically provided on the nameplate of the device:
1.
2.
3.
4.
5.
Device type (Induction motor, DC motor, etc.)
Name of manufacturer
Rated voltages and frequency
Rated currents and VA
Rated speed and horsepower
(Horsepower is a unit of power, 1 hp = 746 W)